The present invention relates to an oxygen concentration detector for detecting the oxygen concentration in a medium such as the oxygen concentration (air-fuel ratio) in an exhaust gas of an internal combustion engine, or more in particular to a temperature control apparatus for the detector which is capable of satisfactorily detecting any air-fuel ratio more than a stoichiometric air-fuel ratio by use of an oxygen sensor of a critical current type current-limiting type.
A conventional apparatus of this type is disclosed in Japanese Patent Laid-Open specification No. 192852/82. Since the critical current characteristic of an oxygen concentration sensor of a critical current type changes with the temperature thereof, the sensor is required to be controlled within a predetermined activating range of temperature. In the sensor disclosed in the above Japanese Laid-Open specification, the sensor temperature is detected and controlled to a constant level by taking advantage of the fact that the internal electric resistance of the sensor changes with temperature. An AC voltage is used for measuring the internal resistance, while a DC voltage is used for measuring the critical current (that is, the oxygen concentration). Specifically, according to the embodiment shown in FIG. 14 of the Laid-Open specification, the critical current and the internal resistance are measured alternately in time division. In the case where the AC voltage is used for measuring the internal resistance, however, the calculation of the sensor temperature requires the processing of a complicated formula, and because of dependence on the frequency and duty factor of the AC voltage, an expensive peripheral device is required for high-precision detection, thereby leading to the disadvantage of high cost.